1. Technical Field
The present invention relates to an apparatus for monitoring an operation state of a battery pack composed of a plurality of cells which are mutually connected in series, and in particular to an apparatus for monitoring an operation state of a battery pack by detecting the voltage of the plurality of cells.
2. Related Art
Recently, battery packs, as represented by lithium batteries, having a plurality of cells mutually connected in series are more frequently used. Usually, such a battery pack is used in combination with an apparatus for monitoring an operation state (e.g., presence/absence of a failure) of the plurality of cells (hereinafter referred to as “battery-state monitoring apparatus”). The information used for monitoring the operation state of the battery pack includes voltage (called “cell voltage”) information. Thus, depending on the way of detecting the cell voltages, battery-state monitoring apparatuses using different methods are available. As generally known, these methods include a method based on comparators or a method based on total-cell-voltage detection.
For example, the method based on comparators is suggested by a published patent document JP-A-H10-023674. This patent document describes an apparatus in which each of a plurality of cells connected in series is provided with a comparator. In each of the comparators, it is determined whether or not the corresponding cell voltage is higher (lower) than a predetermined voltage. Thus, overvoltage (low voltage) of the cell is detected. In this apparatus, the outputs of the comparators are ORed to detect a state where a failure has occurred in any one of the cells. This patent document suggests a method in which, if a failure occurs, the individual cells are disconnected from other devices.
According to the method based on total-cell-voltage detection, a cell voltage of each cell is detected, and the state of each cell is monitored based on the detected voltage itself. Since the cell voltages of the cells are detected one by one in the method based on total-cell voltage detection, the method enables highly accurate cell voltage detection and precise cell voltage management.
The method based on comparators as described in the above patent document enables constant high-speed detection of a failure of a cell. However, since the outputs of the comparators are ORed, there is a problem that the cell having a failure cannot be located.
In the method based on total-cell-voltage detection, the cell voltage of each cell can be detected with high accuracy and thus the cell having a failure can be located. However, the cell voltages of the individual cells have to be detected every time monitoring is carried out. Thus, it takes a long time before all of the cell voltages are detected. Therefore, this causes overshoot in which a cell voltage exceeds a predetermined value, or causes undershoot in which a cell voltage is below the predetermined value, raising a problem such as of affecting the life of the cells.